Photovoltaic (PV) modules generally contain solar cells sealed by an encapsulant which are laminated to a superstrate and a backsheet. The backsheet for PV modules generally consists of a tri-layer laminate structure such as polyvinyl fluoride (PVF; e.g., Tedlar*)-polyethylene terephthalate (PET)-PVF laminate (i.e., TPT laminate). PVF layers provide weathering resistance, while PET provides mechanical strength, dielectric insulation, and water vapor resistance, all desired properties of a PV module backsheet. However, PVF is a fluoropolymer that is not only expensive but is also not friendly to the environment, and difficult to manufacture. PET can typically form low cost plastic films with good mechanical, electrical and water vapor barrier properties, but PET film is susceptible to hydrolysis when exposed to high humidity and hot environments during long term use. Therefore, the performance of the entire backsheet may deteriorate during the lifetime of the PV module. In addition, manufacturing the TPT laminate requires separate adhesive layers to bond the PET and PVF layers together. Such adhesive bonding is typically not of sufficient strength for PV module applications to last the useful life of the module, and the bonding strength can also deteriorate during long-term outdoor exposure of the backsheet, which can lead to interlayer delamination issues.
Besides TPT type structures, the backsheet can have a TPE structure that comprises a fluoropolymer layer (T), a PET layer (P), and a polyethylene layer (E). The TPE backsheet can be utilized to reduce the usage of fluoropolymer in the entire backsheet structure. However, such a backsheet can still require steps to make the individual layers and an additional layer-to-layer lamination process using adhesive layers between the PET layer and the fluoropolymer layer and between the PET layer and the polyethylene layer. As a result, the overall manufacturing cost can be relatively high and the product can also suffer interlayer delamination risk due to insufficient adhesive bonding between the layers during long-term outdoor use.
Polycarbonate based sheets can improve some of the properties that are lacking in a PVF-PET-PVF backsheet. For example, 1,1 bis(4-hydroxy-3-methylphenyl)cyclohexane (dimethyl bisphenol cyclohexane, DMBPC) based polycarbonate copolymer (e.g. Lexan* DMX resins, commercially available from SABIC Innovative Plastics) can provide comparable moisture resistance and better water vapor barrier than bisphenol-A (BPA) based polycarbonate (PC) polymers as shown in U.S. Pat. No. 4,304,899, but articles made from DMBPC polycarbonate copolymers generally suffer from inadequate impact resistance. U.S. Patent Application No. 2009/0176946 discloses a blend of a DMBPC polycarbonate copolymer and BPA polycarbonate to balance pencil hardness and impact resistance. U.S. Patent Application No. 2008/0254299 discloses a composite comprising a DMBPC polycarbonate containing top layer and a BPA polycarbonate based second layer where the formulations therein were optimized for achieving a better pencil hardness of the DMBPC polycarbonate containing top layer. U.S. Pat. No. 7,521,119 discloses DMBPC based polycarbonate copolymers that exhibit better polymer degradation resistance than BPA polycarbonate when both are exposed to ammonia-rich environments such as would be found in rural areas utilizing PV energy systems.
A common environmental condition that many PV modules can be subject to is the combination of ambient heat and high humidity, for which hydrothermal resistance of the PV packaging materials is desired. Thus, there is a need for optimized polycarbonate formulations that can be effectively used in PV module applications.